Turbofan engines have proven to be desirable for aircraft that travel at subsonic speeds. A turbofan engine utilizes energy from a jet stream of gas to turn a turbine shaft. The shaft turns a ducted fan assembly whose blades move a mass of air, providing a thrust that propels the aircraft. The thrust is varied by changing the pitch of the fan blades. During takeoff, when high thrust is required, the pitch of the blades is adjusted to produce maximum engine torque. At cruise, when lower speeds are required, the blade pitch is adjusted to provide optimum fuel efficiency. During landings, the pitch is adjusted to produce a reverse thrust, which brakes the aircraft. Among the advantages offered by variable pitch fan blades, fuel efficiency of the turbofan engine is increased since blade pitch can be varied to cater to ever-changing flight conditions. It is claimed that fuel consumption can be increased by as much as twelve percent by adjusting the pitch of the blades. Further, the reverse thrust capability permits the elimination of a thrust reversal mechanism. This offsets to a certain degree the weight and complexity introduced by the control system that varies blade pitch.
The fan blades are adjusted by an actuation system including a pitch-change gearbox that is located in the hub of the fan and rotated by the fan shaft. The gearbox has a control shaft that is rotated relative to the fan hub. Blade pitch is changed in one direction when the control shaft is rotated faster than the fan hub, and it is changed in the opposite direction when the control shaft is rotated slower than the fan hub.
The control shaft can be rotated by an electric motor such as the two-phase ac motor disclosed in Johnson et al. U.S. Pat. No. 3,900,274. Excitation power is supplied to the motor by two single phase generators, displaced by 90 degrees. Excitation polarity of the field coils of one of the generators is reversible in order to provide the motor with two phase power of reversible sequence.
This motor is mounted for rotation with the hub; therefore, both the rotor and stator of the motor are subjected to rotational stresses. Further, the power windings of the generators are in the same rotating frame of reference as the power windings of the motor; therefore, the generators cannot generate power to change blade pitch in the event the fan shaft stops rotating. As a consequence, the blades cannot be feathered during an in-flight engine failure.